Frequency control circuit for tuning a resonant circuit of an untethered device

ABSTRACT

An untethered device, configured to inductively couple to a source device, includes a tunable resonant circuit having a resonance frequency and configured to generate a supply voltage for the untethered device in response to a varying magnetic field produced by the source device. The tunable resonant circuit includes an inductive coil comprising a center tap and a capacitive circuit coupled to the inductive coil. The capacitive circuit includes an anti-series arrangement of varactor diodes that behave as a capacitance when placed in reverse bias. A frequency control circuit is coupled to the tunable resonant circuit and includes a control voltage source coupled to the center tap of the inductive coil. The control voltage source produces a control signal to place the varactor diodes in reverse bias and to change the capacitance of the capacitive circuit, thereby effecting a change in the resonance frequency of the tunable resonant circuit.

The present invention relates generally to communication between anuntethered device and a sensing system and, more particularly, tosensing systems and methods that employ an untethered stylus as a userinput implement.

BACKGROUND

Personal computing systems of varying type and configuration typicallyprovide one or more user interface devices to facilitate userinteraction with such computing systems. Well known user interfacedevices include a keyboard, mouse, trackball, joystick, and the like.Various types of personal computing devices, such as tablet PCs, providea pen apparatus that can be manipulated by the user, much in the sameway as a pencil or ink pen.

Conventional computing devices that provide for user input via a pen orother pointer implement typically employ an electromagnetic inductivesystem. The electromagnetic inductive system usually comprises anelectromagnetic pen or pointer apparatus and a digitizer in the form ofa tablet. Changes in pen location relative to the digitizer's sensingsurface are detected and location computations are made to determine thecoordinates of the pen.

SUMMARY OF THE INVENTION

The present invention is directed to systems and methods for enhancingcommunication between an untethered device and a sensing system.According to embodiments of the present invention, an untethered deviceis configured to inductively couple to a source device. The sourcedevice is configured to generate a varying magnetic field having a fixedfrequency. The untethered device includes a tunable resonant circuithaving a resonance frequency and configured to generate a supply voltagefor the untethered device in response to the varying magnetic field. Thetunable resonant circuit includes an inductive coil comprising a centertap and a capacitive circuit coupled to the inductive coil.

The capacitive circuit includes an anti-series arrangement of varactordiodes that behave as a capacitance when placed in reverse bias. Afrequency control circuit is coupled to the tunable resonant circuit andincludes a control voltage source coupled to the center tap of theinductive coil. The control voltage source produces a control signal toplace the varactor diodes in reverse bias and to change the capacitanceof the capacitive circuit, thereby effecting a change in the resonancefrequency of the tunable resonant circuit.

The anti-series arrangement of varactor diodes preferably provides for atotal capacitance, C_(t), that is substantially AC voltage amplitudeindependent except for a second order effect. The control signalproduced by the control voltage source preferably changes thecapacitance of the capacitive circuit so that the resonance frequency ofthe tunable resonant circuit is tuned to the fixed frequency of themagnetic field.

The inductive coil includes first and second contacts at respectivefirst and second ends of the inductive coil. A first varactor diodearrangement is preferably coupled between the first end of the inductivecoil and ground. A second varactor diode arrangement is preferablycoupled between the second end of the inductive coil and ground. In someimplementations, each of the first and second varactor diodearrangements includes a multiplicity of varactor diodes. For example,each of the first and second varactor diode arrangements may include amultiplicity of parallel connected varactor diodes.

In some implementations, the control voltage source may include anoperational amplifier having a negative input, a positive input, and anoutput. The output of the operational amplifier may be coupled to thecenter tap of the inductive coil. The positive input of the operationalamplifier may be coupled to ground, and the negative input of theoperational amplifier may be coupled to voltage source circuitry.

In other implementations, the positive input of the operationalamplifier may be coupled to ground, and the negative input of theoperational amplifier may be coupled to a first voltage source and asecond voltage source. The first voltage source may provide a biasvoltage and the second voltage source may provide a slowly varying ACvoltage relative to the resonance frequency of the tunable resonantcircuit.

In accordance with other embodiments, methods of the present inventionmay be implemented involving an untethered device configured toinductively couple to a source device, the source device configured togenerate a varying magnetic field having a fixed frequency. Methods ofthe present invention may involve generating, by a tunable resonantcircuit of the untethered device, a supply voltage for the untethereddevice in response to the varying magnetic field. The tunable resonantcircuit may include an inductive coil coupled to a capacitive circuit,the capacitive circuit comprising an anti-series arrangement of varactordiodes that behave as a capacitance when placed in reverse bias.

Methods of the invention may further involve producing a control signaland applying the control signal to a center tap of the inductive coil toplace the varactor diodes in reverse bias and to change the capacitanceof the capacitive circuit, thereby effecting a change in the resonancefrequency of the tunable resonant circuit. The anti-series arrangementof varactor diodes preferably provides for a total capacitance, C_(t),that is substantially AC voltage amplitude independent except for asecond order effect.

Producing the control signal may involve receiving first and secondvoltage signals and producing the control signal using the first andsecond voltage signals. The first voltage signal may be a bias voltageand the second voltage signal may be a slowly varying AC voltagerelative to the resonance frequency of the tunable resonant circuit.

Changing the capacitance of the capacitive circuit is preferablyeffected using the control signal. For example, changing the capacitanceof the capacitive circuit may be effected using the control signal sothat the resonance frequency of the tunable resonant circuit is tuned tothe fixed frequency of the magnetic field.

In various embodiments, the source device may include an RFID reader andthe untethered device may include an RFID tag. In other embodiments, thesource device includes a digitizer and the untethered device isconfigured as a stylus.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a location sensing system that includes anuntethered stylus and a location sensing device in accordance withembodiments of the present invention;

FIG. 2 is a diagram of various components of a location sensing devicethat cooperates with a stylus in accordance with embodiments of thepresent invention;

FIG. 3 is a diagram of an apparatus for generating an excitationmagnetic field which is received by a stylus in accordance withembodiments of the present invention;

FIG. 4 is an illustration of various components of a stylus implementedin accordance with embodiments of the present invention;

FIG. 5 shows a schematic model of a parallel coil-capacitor circuit thatmay be incorporated in a stylus or other device, such as an RFID tag, inaccordance with embodiments of the present invention;

FIG. 6 is a block diagram of one illustrative implementation thatprovides for continuous tuning of resonant circuitry of an untethereddevice, such as a stylus or RFID tag, in accordance with embodiments ofthe present invention;

FIG. 7 is a schematic of circuitry of a source device and an untethereddevice in accordance with embodiments of the present invention;

FIG. 8 is a block diagram of a frequency control circuit in accordancewith embodiments of the present invention;

FIG. 9 is a plot of cos φ versus normalized resonancefrequency_(fre)/f_(s) for different R₁C time constants in connectionwith the circuitry shown in FIG. 7, the plot of FIG. 9 showing a desiredzero value at the imposed source frequency f_(s) in accordance withembodiments of the present invention;

FIG. 10 is a schematic of circuitry for controlling the resonancefrequency of inductive-capacitive (LC) circuitry of an untethered devicein accordance with embodiments of the present invention;

FIGS. 11 and 12 show the results of changing a control voltage, V_(c),on the resonance frequency of LC circuitry shown in FIG. 10;

FIG. 13 is a block diagram of components that provide for self-tuning ofthe frequency of a drive source that inductively couples to an LCcircuit of an untethered device, such as a stylus or RFID tag, inaccordance with embodiments of the present invention;

FIG. 14 is a schematic of circuitry that may be incorporated in thesource device and untethered device shown in FIG. 13;

FIG. 15 is a plot of capacitor voltage and input impedance phase as afunction of frequency in connection with the circuitry depicted in FIGS.13 and 14; and

FIG. 16 is a plot of the phase of source device input impedance, Z_(in),for different values of k in connection with the circuitry depicted inFIGS. 13 and 14.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description of the illustrated embodiments, referenceis made to the accompanying drawings which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that the embodimentsmay be utilized and structural changes may be made without departingfrom the scope of the present invention.

The present invention is directed to methods and systems for effectingcommunication between an untethered device and a sensing system.Embodiments of the present invention provide for enhanced communicationof information, such as analog and/or digital information, between theuntethered device and the sensing system. Exemplary embodiments of theuntethered device include a stylus or RFID tag. Exemplary embodiments ofthe sensing system include a device locating sensor or touch locationsensor, such as a digitizer, or a RFID reader. Other examples ofuntethered devices include game pieces that interacts with a game boardor other structure that incorporates a sensing system. It is understoodthat methods and systems in accordance with the present invention may beimplemented in a wide range devices and systems other than thoseparticularly described herein.

Embodiments of the present invention are directed to tuning of resonantcircuitry provided at an untethered device, such as a stylus or RFIDtag. Implementations of such embodiments provide for optimal continuoustuning of resonant circuitry in an untethered device, such as byextracting an error control signal that can be used in a feedbackmechanism to self-tune the resonance frequency of inductive-capacitivecircuits. In this manner, the need for time consuming hand tuning ofindividual circuits at the manufacturing level is eliminated, andoptimal performance under varying external conditions, such astemperature, is achieved.

Other embodiments of the present invention are directed to controllingthe resonance frequency of LC circuits in an untethered device using acontrol voltage to change the capacitance, C, of this LC circuit. Anautomatic mechanism using feedback according to such embodimentsprovides for precise and continuous tuning of the capacitor value, C, inorder to ensure optimal circuit performance. Such a mechanism obviatestime consuming and expensive manual tuning of component values andsubsequent detuning because of slowly changing conditions.

According to further embodiments of the present invention, methodologiesprovide for self-tuning the frequency of a source device, so as tooptimize an induced voltage over a capacitor in an LC circuit of anuntethered device. Such embodiments provide methodologies forself-tuning the frequency of a driver that inductively couples to the LCcircuit of an untethered device so that a maximum voltage is developedover the capacitor C in the LC circuit.

Embodiments of an untethered stylus of the present invention may beimplemented in the context of a location sensing system, embodiments ofwhich are illustrated in FIGS. 1-3. According to the embodiments shownin FIGS. 1-3, a location sensing system 10 includes a stylus 12 thatinteracts with a sensing device 11. The sensing device 11 includes alocation sensor 14, such as a digitizer. The stylus 12 is preferablyconfigured as a tetherless or cordless implement that does not have abattery. Rather, the stylus 12 derives power from a magnetic fieldgenerated by the sensing device 11. Although preferred embodiments of anuntethered stylus do not include a battery, some embodiments may employa battery, such as a rechargeable battery that is recharged from energyderived from the magnetic field of the drive signal. A battery may beused to provide power to various circuits of the stylus, such as amodulator or pressure sensor (e.g., tip or eraser pressure sensor).

The sensing device 11 is shown to include a drive loop or coil 18coupled to drive loop electronics 16 that cooperate to generate amagnetic field, which may be a continuously varying magnetic field.Drive coil 18 may comprise one or more coils or loops. The stylus 12,having derived power from the magnetic field emanating from the drivecoil 18, broadcasts a signal from which stylus location and status maybe determined by the sensing device 11.

The stylus 12 is preferably configured to include one or moreuser-actuatable buttons or switches, such as those commonly employed toimplement various mouse functions (e.g., right and left mouse buttons).The tip of the stylus 12 may incorporate a pressure sensor from whichapplied pressure can be resolved and transmitted to the sensing device11. Eraser functionality may also be incorporated in the form of aswitch or pressure sensor at the stylus end opposite the tip.

Sensor interface electronics 20 is coupled to the sensor 14 andfacilitates measurement of signals developed at the sensor 14 inresponse to signals broadcast by the stylus 12. According to oneconfiguration, the sensor 14 includes a digitizer that incorporates adetection grid and electronics as is known in the art. For example, sucha detection grid may include pairs of position resolving conductors eachof which forms one or more differential coil elements in the sensor 14,with each conductor pair receiving a magnetic signal transmitted by thestylus 14. An illustrative example of a digitizer having such adetection grid configuration, elements of which may be employed in alocation sensor system of the present invention, is disclosed in U.S.Pat. Nos. 4,786,765; 5,218,174; 5,633,471; 5,793,360; 6,667,740; and7,019,672; which are hereby incorporated herein by reference.

According to another configuration, the sensing device 11 mayincorporate a sensor 14 that effectively incorporates a digitizer and atouch-sensitive sensor. The digitizer, according to this configuration,allows the location and status of the stylus 12 to be determined. Thetouch-sensitive sensor allows the location of a finger touch to bedetermined. This configuration allows a user to use either the stylus 12or a finger to indicate a desired location on a computer display, aswell as determine the location and status of the stylus 12.

The touch-sensitive sensor 14 typically includes a matrix thatcapacitively couples to the stylus 12 and/or a finger. In thisconfiguration, the sensor 14 of the sensing device 11 is preferably madeup of a series of transparent conductors placed upon a glass or plasticcover that can be placed in front of an LCD display. One side of theglass or plastic sheet has conductors in the X direction, and theopposite side has conductors in the Y direction. Examples of suitabletouch sensitive sensors 14 are disclosed in commonly owned U.S. Pat.Nos. 6,133,906 and 6,970,160, in commonly owned U.S. PublishedApplication No. 2005/0083307, in U.S. Pat. Nos. 6,762,752 and 6,690,156,and in U.S. Published Application No. 2004/0095333, each of which ishereby incorporated herein by reference.

An embodiment that incorporates a digitizer and touch-sensitive sensoradvantageously allows a user to point a stylus at a computer display andhave the location and status of the pointing device determined and, whena finger is used to point at the display device, allows for thedetermination of the location of a finger touch at the display device.The dual use aspects of this embodiment of a sensing device 11 make itparticularly useful in tablet PC applications.

For example, a digitizer arrangement allows a user to use a stylus toinput information, indicate operations the user wants to take, and writeor draw on the display. The touch-sensitive sensor allows the user to“type” information onto a virtual keyboard on the display screen, forexample. This would allow the vendor of the computing system, in which adual touch location sensor system of the present invention isimplemented, to eliminate the keyboard and the associated bulk itrequires. It is understood that a digitizer and a touch-sensitive sensorneed not be implemented together in all configurations, but inclusion ofboth sensing devices provides for enhanced user interaction with acomputing system that incorporates a sensing system 10 of the presentinvention.

According to one embodiment, the drive coil 18 may be constructed ofwire, such as 36 gauge wire, looped several times (e.g., 4 times) aroundthe periphery of the frame of sensing device 11. In one implementation,the drive coil 18 may have an inductance of about 21 μH and an impedanceof about 14 Ohms at 100 kHz. The drive coil 18 is connected to a signalgenerator of the drive loop electronics 16. The signal generator may beconfigured to produce 200 periods of a 100 kHz sine wave signal gated at250 Hz. The signal generator may, for example, produce an output signalof 0.4 V_(pp), resulting in approximately 28 mA of current that flows inthe drive coil 18.

FIG. 3 is a simplified illustration of drive coil 18 and a signalgenerator 17 that cooperate to generate a magnetic excitation field. Inthis illustrative example, one or more coils are preferably arranged inthe plane of the location sensor. A sinusoidal current is produced bythe signal generator 17 with peak magnitude A₁ at radian frequency ω₁and is applied to the rectangular coil 18.

The stylus 12 is configured to collect energy from the magnetic fieldgenerated by drive coil 18/drive loop electronics 16 using a tankcircuit. The tank circuit is preferably tuned to resonate at thefrequency that the drive coil 18 is driven. In this illustrativeexample, the frequency is set at 100 kHz. The tank circuit of the stylus12 builds amplitude during the burst produced by the drive coil 18 andthen gradually loses signal amplitude after the drive coil 18 is turnedoff. The time associated with the exponential charging and dischargingof the resonant tank circuit of the stylus 12 is determined by thecapacitive and inductive elements in the tank circuit. Matching of thetank circuit's resonance frequency and the drive signal frequency ispreferably accomplished using one or more of the techniques describedhereinbelow.

Referring again to FIG. 1, the sensor interface electronics 20 ispreferably connected to the sensor 14 via a shielded connector. Thesensor interface electronics 20 includes circuitry for measuring thesignal levels present on the individual traces of the sensor 14, and istypically configured to reject as much noise as possible.

As is shown in FIG. 2, an envelope detector circuit 30 of the sensorinterface electronics 20 is configured to detect signals developed onindividual traces of the sensor 14. The signals output by the envelopedetector circuit 30 are digitized by use of analog-to-digital (A/D)converters 32. Each trace of the sensor 14 may have a dedicated A/Dconverter 32. Alternatively, two or more traces may share a common A/Dconverter 32 via a switch having a sufficient switching frequency. Theenvelope detector circuit 30 is configured to provide sufficient gain tomake the resultant signal match the requirements of A/D converters 32.The envelope detector circuit 30 may be configured to generate a signalhaving the same shape as an imaginary line describing the upper bound ofthe sensor signal. In such a configuration, the envelope detectorcircuit 30 effectively transforms the 100 kHz signal into a DC or lowfrequency signal that is more readily digitized. The envelope detectorcircuit 30 preferably incorporates one or more synchronous demodulators.

A processor 22 is coupled to the drive loop electronics 16, sensorinterface electronics 20, and a communications interface 24, as is shownin FIG. 1. The processor 22 coordinates the operations of drive loopelectronics 16 and sensor interface electronics 20, and is configured todetermine stylus/finger location and stylus status. Stylus/fingerlocation and stylus status determinations may be made by the processor22 using known approaches, such as those discussed in the patentreferences incorporated herein by reference. In one embodiment,processor 22 determines stylus/finger location and stylus status inaccordance with the methodologies disclosed in commonly owned U.S.patent application Ser. No. 11/557,829, entitled “Touch Location SensingSystem and Method Employing Sensor Data Fitting to a Predefined Curve,”filed on Nov. 8, 2006, which is hereby incorporated herein by reference.

The location and status information computed by the processor 22 iscommunicated to a computer and/or display 26 via a communicationsinterface 24. The communications interface 24 may be configured as anRS-232 or USB interface, for example. The processor 22 may be configuredto drive a display 26 directly. Alternatively, a computer 28 may becoupled to the communications interface 24 and receive the location andstatus information from the processor 22, and drive its display. Theprocessor 22 or computer 28 may be configured to control cursorvelocity, momentum and other factors to enhance the user experience withthe sensing system 11.

Referring now to FIG. 4, there is shown an embodiment of an untetheredstylus 12 of the present invention that may be implemented in thecontext of a location sensing system as described above or other sensingsystem known in the art. In accordance with the embodiment shown in FIG.4, a stylus 12 houses electronics 52, which includes frequency sensitiveLC circuitry, and a coil 54 wrapped around a ferrite cylinder 53. Theferrite cylinder 53 serves to increase signal amplitude. An appliedharmonic magnetic field produced at the surface of the location sensor(e.g., digitizer) or a display, for example, couples flux through theferrite cylinder 53 and thus to the coil 54 when the stylus 12 is placedin the applied field.

The ferrite coil arrangement 56 resonates with a separateparallel-connected capacitor of the electronics 52 and is tuned to theexcitation field frequency. In various embodiments, circuitry 55 isprovided in the untethered stylus 12 to provide for self-tuning of theresonance frequency of the frequency sensitive LC circuitry inaccordance with one or more techniques described herein. In otherembodiments, the frequency of a source voltage produced by the sourcedevice that generates the excitation field is adjusted so that the phaseof an input impedance of the source device is substantially zero,thereby maximizing a voltage across the capacitor of the frequencysensitive LC circuitry of the untethered stylus 12.

The parallel coil-capacitor combination is connected between the stylustip 57 and the stylus shield 59. The shield 59 may form part of, orotherwise be connected to, the stylus housing so that it can be touched,and therefore grounded, by a user's hand when held. The shield 59 may besituated to extend over the circuitry region of the stylus 12, andpreferably has a discontinuous shape, such as a “C” shape, so as toavoid eddy currents that could otherwise arise in a closed loop shieldarrangement.

The stylus tip 57 couples capacitively to the location sensor from whichlocation information is derived. To provide stylus status information,the ferrite coil arrangement 56 powers the electronics 52 whichamplitude modulates the stylus tip voltage at the reference frequency orfrequencies. The frequency of the oscillations is changed to reflect thestylus status, such as switch closures or tip pressure changes.

Alternatively, the invention may be implemented with magnetic-sensingdigitizer systems as are known in the art. An untethered magnetic stylusis similar to the capacitive stylus shown in FIG. 4, except the resonantcircuit comprising ferrite coil arrangement 56 and separateparallel-connected capacitor of the electronics 52 need not be connectedto tip 57 nor to a shield 59. Untethered magnetic styluses are wellknown in the art, and are described in previously incorporated U.S. Pat.Nos. 4,786,765; 5,633,471; 5,793,360; 6,667,740, and 7,019,672.Embodiments of the present invention that are implemented using anuntethered magnetic stylus may employ a location sensor that includesmultiple drive loops as disclosed in the referenced patents. In suchembodiments, a separate sensing grid and separate drive loops need notused. Rather, each of the drive loop coils is alternately coupled totransmitting circuitry and then to receiving circuitry to alternatelytransmit and receive from one of multiple drive loop coils that areplaced in the active area, typically under the display.

FIG. 5 shows a schematic model of a parallel coil-capacitor circuit thatfacilitates an enhanced understanding of the present invention. Theparallel coil-capacitor circuit shown in FIG. 5 (i.e., tank circuit) maybe incorporated in a stylus as part of, or coupled to, resonancefrequency tuning circuitry in accordance with embodiments of the presentinvention. FIG. 5 shows a capacitor C1 connected in parallel with a coil54 to resonate at the excitation frequency or the transmitted frequency.The voltage developed across the coil 54, which is shown modeled asvoltage generator 61, is coupled to the stylus tip 57 and thencapacitively coupled to the location sensor, such as sensor 14 shown inFIG. 1. The voltage developed across the resonating coil 54 ispreferably modulated with one or a combination of known techniques. Anadded ferrite cylinder 53 about which coil 54 is preferably wrapped, asshown in FIG. 5, has the effect of increasing the magnetic flux B andsignal coupled by the drive coil of the location sensor to the receivingcoil 54 of the stylus 12.

The capacitance value of capacitor C1 shown in FIG. 5 is selected suchthat the capacitance, C, of capacitor C1 resonates with the coilinductance, L, at the excitation angular frequency ω so that there is novoltage drop across the LC combination. Two different voltages in thiscircuit can be considered. The first voltage of consideration is thevoltage V (shown in terms of voltage source 61) that develops across thecoil 54 through magnetic induction. It is well understood that thisvoltage 61 is basically equal to the number of stylus coil turns N timesthe coil cross section A times the rate of change of the magnetic fluxdensity passing through the ferrite cylinder, which is given byV=N*A*dB/dt.

The second voltage of consideration is the voltage that develops acrossthe capacitor C1. This voltage V_(C) is also the stylus tip voltage.From basic circuit analysis at resonance, it follows that:V_(C)=V/(ωRC)=V(ωL/R) with the quantity 1/(ωRC)=(Lω)/R defined as theresonant circuit quality factor Q, where ω is expressed in terms ofradians per second. This second voltage may be modulated for purposes ofcommunicating stylus status data to a location sensor.

With continued reference to FIG. 5, one approach to transmitting stylusstatus information in addition to stylus position information is throughaddition of a second capacitor C2 connected to the first capacitor C1through a switch 16. Opening and closing the switch 16 causes theresonance frequency of the coil-capacitor combination 54/C1 to change.This change may be detected by observing a change in phase of the stylustransmitted frequency or though a transient frequency change caused whenthe drive coil current is turned off.

This method of data transmission, however, is not suitable for a styluspowered by a constantly varying magnetic field and capacitively coupledto the digitizer. Constant excitation does not allow a transientmeasurement of the stylus resonance, and phase modulation is difficultto detect as the phase of the digitizer received signal variesdramatically as the stylus is moved across the location sensor (e.g.,digitizer). Frequency modulation of an amplitude-modulated signal usingmultiple reference frequencies generated at the stylus, for example,removes these difficulties. The location sensor may be configured todemodulate the amplitude modulation and detect the reference frequenciesof the modulation.

FIG. 6 is a block diagram of one illustrative implementation thatprovides for continuous tuning of resonant circuitry of an untethereddevice 12, such as a stylus or RFID tag. According to this illustrativeembodiment, the untethered device 12 implements a method of extractingan error control signal that can be used in a feedback mechanism toself-tune the resonance frequency of LC circuitry of the untethereddevice 12.

According to the implementation shown in FIG. 6, an untethered device 12includes a tunable resonant circuit 62, which includes inductive andcapacitive circuit components. The tunable resonant circuit 62 has aresonance frequency that can be continuously adjusted to match thefrequency of the source device 60. The source device 60 typicallygenerates a varying magnetic field having a fixed frequency.

The untethered device 12 is further shown to include a frequency controlcircuit 64, which is coupled to the tunable resonant circuit 62 via acontrol line 66. The frequency control circuit 64 is configured toproduce a control signal, S_(c), using signals developed within thetunable resonant circuit 62. In this illustrative example, voltagesignals V₁ and V₂, developed within the tunable resonant circuit 62, areextracted for use in tuning the tunable resonant circuit 62. The controlsignal, S_(C), generated by the frequency control circuit 64, is useableto adjust the resonance frequency of the tunable resonant circuit 62 tomatch the fixed frequency of the source device 60.

Turning now to FIG. 7, there is shown a schematic of circuitry of asource device 60 and an untethered device 12 in accordance withembodiments of the present invention. In configurations in which theuntethered device 12 is implemented as an RFID tag, the L₁-C circuit 72,74 is referred to as the “tag,” while the L₀ inductance component 71 ispart of a “reader,” with fixed frequency f_(s). It is understood thatthe circuitry shown in FIG. 7 and in other Figures may be employed insystems and devices other than those that employ untethered styli andRFID tags.

In conventional implementations, the components L₁ 72 and C 74 arefixed, such that the resonance frequency, f_(res), of the L₁-Ccombination 72, 74 is defined as:

$f_{res} = \frac{1}{2\pi \sqrt{L_{1}C}}$

The resonance frequency, f_(res), of the L₁-C combination 72, 74 is notexactly equal to the source frequency, f_(s). As a result, the voltageV₂ that develops across the capacitor C 74 is lower than desired.Because this voltage, V₂, is often used to power additional circuitry orcommunication electronics, a reduced voltage across the capacitor C 74results in suboptimal performance. A tunable resonant circuit withfrequency control in accordance with the present invention provides animprovement in performance, in that L₁ 72, C 74 or both may beautomatically and continuously tuned so that the resonance frequency,f_(res), of the L₁-C combination 72, 74 becomes exactly equal to thefrequency, f_(s), of the source device 60.

Defining the angular resonance frequency ω_(res)=2πf_(res) and applyingKirchoff's laws to the circuit in FIG. 7 leads to the following:

$\frac{V_{2}}{V_{1}} = {{\frac{\left( {\omega^{2} - \omega_{res}^{2}} \right)}{\omega_{res}^{2}}\frac{R_{2}}{R_{1}}} - {{j\omega}\; R_{2}C}}$

so that the phase angle, φ between V₁ and V₂ is given by:

$\phi = {{\arctan \frac{\omega_{res}^{2}}{\omega_{res}^{2} - \omega^{2}}\omega \; R_{1}C} = {\arctan \frac{2\pi \; {fR}_{1}C}{\left( {1 - \left( {f/f_{res}} \right)^{2}} \right)}}}$

Voltage signals V₁ and V₂ developed as shown in FIG. 7 are preferablyfed to a frequency control circuit, a general embodiment of which isshown in FIG. 6. Using the voltage signals V₁ and V₂ as inputs, thefrequency control circuit produces a control signal that is fed back tothe resonant circuitry which, in response, results in adjustment of theresonance frequency of the untethered device 12 to match the frequencyof the source voltage produced by the source device 60.

In general, the control signal generated by the frequency controlcircuit is preferably a signal that is proportional to the voltagesignals V₁ and V₂. The control signal may also be a signal that isproportional to the voltage signal V₂. According to one approach, thecontrol signal is preferably produced by a multiplication of the voltagesignals V₁ and V₂. If V₁ and V₂ are harmonic and have the same frequencybut a phase shift of φ, as defined above, the multiplication results inthe following:

$\begin{matrix}{V_{out} = {V_{1}\cos \; \omega \; t*V_{2}{\cos \left( {{\omega \; t} - \phi} \right)}}} \\{= {\frac{V_{1}V_{2}}{2}\left\{ {{\cos \; \phi} + {\cos \left( {{2\omega \; t} - \phi} \right)}} \right\}}}\end{matrix}$

The first term in the brackets of the resultant equation, cos φ, is thedesired control signal component. The second term, cos(2φt−φ), is adouble frequency signal component that is undesirable. This second termis preferably filtered out of the control signal, which, afterfiltering, can be represented as V_(c) cos φ. Those skilled in the artwill appreciate that various known techniques used in a variety ofmixing and superheterodyning applications may be employed to extract acontrol signal usable to adjust the resonance frequency of the tunableresonant circuit of the untethered device 12 to match the fixedfrequency of the source device 60 in accordance with the presentinvention.

FIG. 8 is a diagram of one implementation of a frequency control circuitin accordance with embodiments of the present invention. The frequencycontrol circuit shown in FIG. 8 includes a multiplier 93 coupled to alow pass filter 95. The voltage signals V₁ and V₂, such as those shownin FIGS. 6 and 7, are fed into the multiplier 93. The output of themultiplier 93, V_(out), is a voltage signal produced by a directmultiplication of V₁ and V₂, such that V_(out)=V₁*V₂.

As discussed above, if V₁ and V₂ are harmonic and have the samefrequency but a phase shift of φ, then V_(out) is calculated accordingto the equation above. This signal, V_(out), is then fed to the low passfilter 95, which may be configured to filter out the double frequencysignal component discussed above or to simply pass only a DC signal. Theoutput of low pass filter 95 is the control signal V_(c) cos φ. It isnoted that both V₁ and V₂ will not vanish near the resonance frequency,f_(res), of the untethered device's tunable resonant circuit, so thatV_(c) will always have an acceptable amplitude.

In low frequency applications (e.g., <1 MHz), the multiplier 93 mayinclude an analog multiplier chip, such as a four-quadrant analogmultiplier model AD633 available from Analog Devices, Inc. For highfrequency applications (i.e., >1 MHz), any suitable mixer may be used.

FIG. 9 is a plot of cos φ versus normalized resonance frequencyf_(res)/f_(s) for different R₁C time constants in connection with thefrequency control circuit shown in FIG. 8. The plot of FIG. 9 clearlyshows a desired zero value at the imposed source frequency f_(s). Thus,the signal V_(c) cos φ can be used as a control signal to force the L₁-Ccircuit shown in FIG. 7 to adjust f_(res) to become equal to f_(s)through a feedback mechanism. FIG. 9 shows that the dynamic range of cosφ is greater for smaller values of R₁ and C. A small value of R₁ isconsistent with the requirement that the resonance circuit has a high Qvalue.

FIG. 10 is a schematic of circuitry for controlling the resonancefrequency of LC circuitry of an untethered device in accordance withother embodiments of the present invention. Control circuitryimplemented in accordance with embodiments encompassed by FIG. 10provides for controlling the resonance frequency of LC circuitry of anuntethered device using a control voltage that changes the capacitance,C, of the LC circuitry.

It is understood by those skilled in the art that inductances (L) andcapacitances (C), as well as other factors, influence the resonancefrequency of a circuit made up of these components. For high qualitycircuits, for example, resonance frequency variation can be verysensitive for even slightly different component values. To avoid timeconsuming and thus expensive manual tuning of component values, and toavoid subsequent detuning because of slowly changing conditions, anautomatic mechanism using feedback in accordance with embodiments of thepresent invention provides for precise continuous tuning of thecapacitor value, C, to ensure optimal circuit performance.

FIG. 10 shows one configuration of a voltage controlled resonancefrequency LC circuit in accordance with embodiments of the presentinvention. In the implementation shown in FIG. 10, a coil 79 representsan inductance L of the circuit, and C₁ 81 and C₂ 82 are varactor diodesthat are always driven in reverse so that they behave as capacitances.An output of an OPAMP 80 is shown coupled to the center tap of the coil79. In this arrangement, the OPAMP output voltage as the center tapvoltage for coil 79 placed both varactor diodes 81, 82 in reverse biaswith the same voltage, so that both V_(b) and V_(c) can control theresonance frequency of the L, C₁, C₂ combination.

The bias voltage, V_(b), is essentially DC and the control voltage,V_(c) is at most slowly varying compared to the resonance frequency ofthe LC circuit. The OPAMP output voltage, V_(o), is given by:

$V_{o} = {{{- \frac{R_{3}}{R_{1}}}V_{c}} - {\frac{R_{3}}{R_{2}}V_{b}}}$

Demanding that varactor diodes C₁ 81 and C₂ 82 are always driven inreverse or that the OPAMP output voltage, V_(o), is always negative, asgiven below:

−V _(s) <V _(o)<0

results in the following allowed operating range for the controlvoltage, V_(c):

${{- \frac{R_{1}}{R_{2}}}V_{b}} < V_{c} < {{\frac{R_{1}}{R_{3}}V_{s}} - {\frac{R_{1}}{R_{2}}V_{b}}}$

When R₁ 70=R₂ 77=R₃ 78, and with V_(b)=0.5 V_(s), the control voltage,V_(c), has the following desirable range:

${- \frac{V_{s}}{2}} < V_{c} < \frac{V_{s}}{2}$

Because of the anti-series arrangement of the varactor diodes C₁ 81 andC₂ 82, an external alternating flux that is coupled by coil 79 willinduce a voltage over coil 79 that will cause varactor diode C₁ 81 to bedriven less in reverse and varactor diode C₂ 82 more in reverse.Assuming a linear varactor capacitance-voltage dependence ΔC=kΔV, thismeans that varactor diode C₁ 81 undergoes a capacitance change +ΔCwhereas varactor diode C₂ 82 undergoes a −ΔC change. With C₁=C₂=C, thetotal capacitance, C_(t), formed by C₁ 81 and C₂ 82 is then given by:

$\frac{1}{C_{l}} = {\frac{1}{C + {\Delta \; C}} + \frac{1}{C - {\Delta \; C}}}$

so that:

$C_{l} = {\frac{C}{2} - \frac{\left( {k\; \Delta \; V} \right)^{2}}{2C}}$

meaning that the total capacitance, C_(t), is virtually AC voltageamplitude independent, except for a second order effect.

In one implementation, a total of 8 varactor diodes as 2 groups of 4parallel diodes may be used to form C₁ 81 and C₂ 82. A ferritecontaining self-inductance (L) coil 79 of 22 mH may be used so that thetotal capacitance, C_(t)=C₁C₂/(C₁+C₂), at V_(b)=7.5 V and V_(c)=0 V isestimated to be about 115 pF. Therefore, C₁ 81 and C₂ 82 each have acapacitance of approximately 230 pF. The sensitivity for such animplementation is around 1 kHz/V.

The circuit in FIG. 10 may be implemented using silicon varactor diodes,such as MVAM115 silicon varactor diodes available from AdvancedSemiconductor, Inc. OPAMP 80 is preferably a low noise OPAMP, such asOP27 Low Noise, Precision Operational Amplifier available from AnalogDevices, Inc.

FIGS. 11 and 12 show the results of changing control voltage, V_(c), onthe resonance frequency. FIG. 11 shows a resonance peak for V_(c)=0volt, at a resonance frequency≈100 kHz. FIG. 12 shows a resonance peakfor V_(c)=5 volt, at a resonance frequency≈95 kHz. The frequency spectrain FIGS. 11 and 12 were obtained by inductively inducing a whitespectrum into coil 79 and measuring the LC circuit response afterchanging V_(c).

Further embodiments of the present invention are directed to self-tuningthe frequency of a drive source that inductively couples to an LCcircuit of an untethered device, such as a stylus or RFID tag, so that amaximum voltage develops over the capacitor, C, in the LC circuit. FIG.13 illustrates an implementation of one such embodiment, in which asource device 60 inductively couples to an LC circuit in an untethereddevice 12. The source device 60 is shown to include a driver circuit 92coupled to drive coil 18, which may include one or more coils. Thedriver circuit 92 may be implemented as part of the drive loopelectronics 16 shown in FIG. 1.

Also shown in FIG. 13 is a phase detector 96 coupled to the drivercircuit 92, the driver circuit 92 further including a drive coilcircuit. A controller 94 is coupled to the phase detector 96 and thedriver circuit 92. The controller 94 may be part of, or coupled to, theprocessor 22 shown in FIGS. 1 and 2.

In operation, the driver circuit 92 and drive coil 18 cooperate togenerate a varying magnetic field. The phase detector 96 detects a phaseof an input impedance of the driver circuit 92 in response to the sourcedevice 60 inductively coupling with the LC circuit of the untethereddevice 12. The controller 94 adjusts a frequency of a source voltageapplied to the driver circuit 92 in response to an output signal of thephase detector 96. The controller 94, which may be implemented using avoltage controlled oscillator, adjusts the source voltage frequency sothat the phase of the input impedance as indicated by the output signalof the phase detector 96 is substantially zero. In this manner,controller adjustment of the source voltage frequency compensates for achange in the resonance frequency, or a parameter of the resonancefrequency, of the resonant LC circuit of the untethered device 12.

FIG. 14 is a schematic of circuitry that may be incorporated in thesource device 60 and untethered device 12 shown in FIG. 13. FIG. 14shows the source device 60 inductively coupled to an LC resonancecircuit of the untethered device 12. The frequency matching methodologyaccording to this embodiment involves detecting the phase of the sourcedevice's input impedance, Z_(in) (looking into the drive coil 84 (L₁)),such as by use of a phase detector, and adjusting the frequency of thesource voltage V 65 until that phase is zero. The output of the phasedetector can then be used in a feedback circuit to drive a voltagecontrolled oscillator (VCO), for example, with V as an output signal.

A variety of available phase detectors may be used to perform phasedetection and comparison. In low frequency applications (i.e. <1 MHz),for example, an analog multiplier, such as the aforementioned AD633, maybe used. In higher frequency applications (i.e., >1 MHz), a passivecomponent, such as a PDP-201 Phase Detector available from SynergyMicrowave Corporation, may be used for phase detection. Other commercialcomponents and implementation may be used that provide similarfunctionality and performance.

The following analysis is provided with reference to FIG. 14. Withsufficient coupling k, the phase of Z_(in) is zero at the frequency thatcorresponds with the maximum voltage V_(c). Applying Kirchoff's laws tothe circuit in FIG. 14 leads to:

$\begin{matrix}{\frac{V_{c}}{V} = {\frac{1}{C} \cdot \frac{k\sqrt{L_{1}L_{2}}}{{\left( {R_{1} + {{j\omega}\; L_{1}}} \right)\left( {R_{2} + {{j\omega}\; L_{2}} - \underset{\omega \; C}{j}} \right)} + {\omega^{2}k^{2}L_{1}L_{2}}}}} & \lbrack 1\rbrack\end{matrix}$

with ω angular frequency, whereas Z_(in) is given by:

$\begin{matrix}{Z_{in} = {R_{1} + {{j\omega}\; L_{1}} + \frac{k^{2}\omega^{2}L_{1}L_{2}}{R_{2} + {{j\omega}\; L_{2}} - \frac{j}{\omega \; C}}}} & \lbrack 2\rbrack\end{matrix}$

FIG. 15 is a plot of capacitor voltage and input impedance phase as afunction of frequency. More particularly, FIG. 15 is a plot of thevoltage ratio V_(c)/V of equation [1] and phase, Z_(in), of equation [2]for the following values of the components shown in FIG. 14: L₁=45 μH,L₂=22 mH, R₁=9Ω, R₂=40Ω, C 87=153.5 pF, and k=0.5. As is clear in FIG.15, the capacitor voltage, V_(c), is maximum near the higher frequencypoint where the phase of the input impedance, Z_(in), is zero.

The source frequency self-tuning methodology described above works wellfor strong coupling or high values of k, where k takes on values between0 and 1 (i.e., 0<k<1). FIG. 16 gives an indication of the phase ofZ_(in) for different values of k. It can be seen from FIG. 16 that thedetection mechanism breaks down when k gets too small because the phaseof Z_(in) no longer goes through zero. The fact that the phase of Z_(in)for k=0.1 fails to go through zero in FIG. 16 may not be entirelyobvious, but this fact can easily be confirmed by reproducing the graphdepicted in FIG. 16 using Equation [2] above.

The foregoing description of the various embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

1. An untethered device configured to inductively couple to a sourcedevice, the source device configured to generate a varying magneticfield having a fixed frequency, the untethered device comprising: atunable resonant circuit having a resonance frequency and configured togenerate a supply voltage for the untethered device in response to thevarying magnetic field, the tunable resonant circuit comprising: aninductive coil comprising a center tap; and a capacitive circuit coupledto the inductive coil, the capacitive circuit comprising an anti-seriesarrangement of varactor diodes that behave as a capacitance when placedin reverse bias; and a frequency control circuit coupled to the tunableresonant circuit, the frequency control circuit comprising a controlvoltage source coupled to the center tap of the inductive coil, thecontrol voltage source producing a control signal to place the varactordiodes in reverse bias and to change the capacitance of the capacitivecircuit, thereby effecting a change in the resonance frequency of thetunable resonant circuit.
 2. The device of claim 1, wherein theinductive coil comprise first and second contacts at respective firstand second ends of the inductive coil, a first varactor diodearrangement coupled between the first end of the inductive coil andground, and a second varactor diode arrangement coupled between thesecond end of the inductive coil and ground.
 3. The device of claim 2,wherein each of the first and second varactor diode arrangementscomprise a plurality of varactor diodes.
 4. The device of claim 2,wherein each of the first and second varactor diode arrangementscomprise a plurality of parallel connected varactor diodes.
 5. Thedevice of claim 1, wherein the control voltage source comprises anoperational amplifier having a negative input, a positive input, and anoutput, the output of the operational amplifier coupled to the centertap of the inductive coil.
 6. The device of claim 5, wherein thepositive input of the operational amplifier is coupled to ground, andthe negative input of the operational amplifier is coupled to voltagesource circuitry.
 7. The device of claim 5, wherein the positive inputof the operational amplifier is coupled to ground, and the negativeinput of the operational amplifier is coupled to a first voltage sourceand a second voltage source, the first voltage source providing a biasvoltage and the second voltage source providing a slowly varying ACvoltage relative to the resonance frequency of the tunable resonantcircuit.
 8. The device of claim 1, wherein the anti-series arrangementof varactor diodes provides for a total capacitance, C_(t), that issubstantially AC voltage amplitude independent except for a second ordereffect.
 9. The device of claim 1, wherein the control signal produced bythe control voltage source changes the capacitance of the capacitivecircuit so that the resonance frequency of the tunable resonant circuitis tuned to the fixed frequency of the magnetic field.
 10. The device ofclaim 1, wherein the source device comprises an RFID reader and theuntethered device comprises an RFID tag.
 11. The device of claim 1,wherein the source device comprises a digitizer and the untethereddevice comprises a stylus.
 12. A method implemented in an untethereddevice configured to inductively couple to a source device, the sourcedevice configured to generate a varying magnetic field having a fixedfrequency, the method comprising: generating, by a tunable resonantcircuit of the untethered device, a supply voltage for the untethereddevice in response to the varying magnetic field, the tunable resonantcircuit comprising an inductive coil coupled to a capacitive circuit,the capacitive circuit comprising an anti-series arrangement of varactordiodes that behave as a capacitance when placed in reverse bias; andproducing a control signal and applying the control signal to a centertap of the inductive coil to place the varactor diodes in reverse biasand to change the capacitance of the capacitive circuit, therebyeffecting a change in the resonance frequency of the tunable resonantcircuit.
 13. The method of claim 12, wherein producing the controlsignal comprises receiving first and second voltage signals andproducing the control signal using the first and second voltage signals.14. The method of claim 13, wherein the first voltage signal comprises abias voltage and the second voltage signal comprises a slowly varying ACvoltage relative to the resonance frequency of the tunable resonantcircuit.
 15. The method of claim 12, wherein the anti-series arrangementof varactor diodes provides for a total capacitance, C_(t), that issubstantially AC voltage amplitude independent except for a second ordereffect.
 16. The method of claim 12, comprising changing the capacitanceof the capacitive circuit using the control signal.
 17. The method ofclaim 12, comprising changing the capacitance of the capacitive circuitusing the control signal so that the resonance frequency of the tunableresonant circuit is tuned to the fixed frequency of the magnetic field.18. The method of claim 12, wherein the anti-series arrangement ofvaractor diodes comprises first and second varactor diode arrangements,the first and second varactor diode arrangements each comprising aplurality of parallel connected varactor diodes.
 19. The method of claim12, wherein the source device comprises an RFID reader and theuntethered device comprises an RFID tag.
 20. The method of claim 12,wherein the source device comprises a digitizer and the untethereddevice comprises a stylus.